Microelectrode neural probes provide a direct electrical interface with the neurons of a biological entity's nervous system. Such neural probes can target the neuronal activity of neurons, enabling researchers and clinicians to better explore and understand neurological diseases, neural coding, neural modulations, and neural topologies.
Moreover, the capabilities for analyzing neuronal activity using neural probes has led to the development of new neuro-therapeutic devices implemented through brain-machine interfaces (BMIs) comprising neural probes. An underlying objective in using BMI devices is to bypass damaged tissue through the BMI so that a patient can regain lost communication and/or control with respect to some aspect of the patient's nervous system. Such devices, for example, may restore physical mobility and/or communicative abilities to patients suffering from paralysis due to spinal chord injury, stroke, brachial plexus injury, or similar types of injuries involving the nervous system. Through a BMI a patient, for example, could directly control a prosthetic limb or wheelchair. Similarly, through a BMI the patient could communicate through an external device controlled by the patient.
Typically, with the BMI, signals corresponding to neural activity are chronically collected from the cortex of the patient's brain. The signals are interpreted and, in turn, signals for effecting a needed therapy are delivered through the interface. Accordingly, an important objective with the BMI is a capability for acquiring, via the interface, sufficiently discernable neural signals. This typically requires that electrodes of the BMI be inserted into the cortical tissue of a patient's brain with the sufficient spatial resolution needed to record action potentials from individual neurons.
Generally, there are two principal types of fixed neural probes that are commonly used for recording neural signals. One type is a neural probe using wire microelectrode neural probes assembled from insulated tungsten wires. The other type is a neural probe using micro-machined electrodes fabricated using conventional integrated circuit (IC) micro-fabrication technologies.
Wire microelectrodes can provide precise signal “firing” information with respect to individual neurons in the cortical and sub-cortical tissues. Micro-wire electrodes typically comprise bundles of tungsten wires having diameters of 50 μm and electro-polished tips, either blunt or sharpened. Fabricating arrays of microelectrodes for chronic use has proved a particular challenge since the arrays are typically assembled from discrete components. Moreover, the array layout, particularly inter-electrode spacing and electrode morphology are usually not uniform because the arrays are constructed from drawn strands of wire.
The second type of fixed neural probe was devised in an attempt to overcome the inherent challenges with respect to the first type by utilizing micro-fabrication and micro-machining techniques typically employed with IC fabrication. This second type of fixed neural probe is fabricated using photolithography to transfer electrode patterns. Silicon electrode shanks can be fabricated through selective etching using impurity etch stops in concert with anisotropic liquid etchants as well as anisotropic dry etches. It also is possible to interpose signal processing circuitry onto the substrate of the probe. Signal recording sites of such probes typically comprise exposed metal pads located on rigid shanks that are connected, via interconnection traces, to output leads or to signal processing circuitry on a monolithic substrate.
Some neural probes are fabricated to include multiple recording sites placed along the length of the shank to enable signal interrogation of cells at varying depths of the neural tissue. The added wiring required, however, can increase the thickness of the electrode shanks up to approximately 160 μm, which increases the risk that insertion of the probe could injure a patient's brain tissue. Another approach is to fabricate the neural probe with internal circuitry to multiplex the electrodes, thereby facilitating added numbers of recording electrodes without undue increases in shank dimensions. The added circuitry, however, can increase the complexity of the fabrication process.
Another concern regarding rigid-substrate, micro-machined neural probes is that the probes are subject to mechanical forces owing to strain that transfers from the mount to the probes as they “float” in neural tissue, thereby creating the risk for reduced reliability of the probe resulting from injury to the tissue. One approach intended to mitigate this risk is to make the substrate of the microelectrode array flexible by utilizing thin-metal electrode sites and enclosing the wiring between polymer materials. The resulting electrode array is completely flexible, thereby providing needed strain relief. However, this design prevents direct insertion of the probe into brain tissue. Instead, with this type of probe, an incision much be first created to effect implantation. This typically results in increased tissue damage. An alternative is to use a complex design of rigid probes with planar electrode sites on the probe shank and a flexible cable to connect electrodes to output ports.
As yet, however, there is not a flexible neural microelectrode array design that provides a good compromise between the best properties of micro-wire electrodes and MEMS fabrication techniques. More generally, there is as yet a need for an electrode array that can achieve high neuronal yield that is highly customizable in terms of geometry/layout, that can mitigate tissue damage during implantation, and that also can be relatively easily and efficiently fabricated in large numbers.